This invention relates to a flow-measuring device, and more specifically to a device for measuring the rate of fluid flow through a completely filled duct of rectangular cross section.
Prior art devices of the type are as shown, for example, in FIGS. 1 and 2. The reference numeral 1 is a duct having a rectangular cross section, 2 is a segmental orifice plate as the primary measuring element, and 3a, 3b, are pressure taps formed in the wall of the duct portions upstream and downstream, respectively, from the orifice plate. As is well known in the art, the segmental orifice plate 2 is secured to inner walls of the duct 1, normal to the streamlines therein, so that the differential pressure created by the flow through the orifice and obtained from the taps 3a, 3b is determined for the measurement of the fluid flow rate by calculation with a known flow coefficient.
Where the straight duct section upstream of the primary element is not sufficiently long, the measuring device having such conventional orifice plates may present the following disadvantages:
A. With an ordinary segmental orifice the flow stream in the duct is deflected toward one side of the channel cross section where the orifice or orifice plate is provided. This will lead to an error in the resulting differential pressure should there remain any adverse effect of non-uniform flow due to the upstream configuration of the duct.
FIG. 3 shows streamlines being deflected toward the orifice or the opening left by the segmental orifice plate 2. Here the main stream directly passes through the orifice, and therefore the differential pressure produced by the orifice plate tends to become lower than that obtained from the flow of uniform velocity distribution.
In FIG. 4 the streamlines are deflected toward the opposite side or against the orifice plate. Because the main stream is baffled and throttled by the plate into the orifice, the resulting differential pressure will often be higher than that obtained with uniform velocity distribution.
Experimental data demonstrating the foregoing tendencies are plotted in FIG. 5. Of the curves representing the relations between the differential pressure produced and the mean flow velocity, the full line indicates the data obtained with uniform velocity distribution, the chain line the data in the case of FIG. 3, and the broken line the data in the case of FIG. 4.
b. Since the segmental orifice plate 2 is installed one-sidely, the downstream flow is deflected accordingly. If the channel has a damper or bend a short distance behind the orifice, the damper effect may decrease or the pressure loss at the bend may increase. Other possible troubles include intensified deflection flow downstream.
The disadvantage (a) of the conventional segmental orifice plate 2 will now be more fully discussed in connection with an actual arrangement wherein the orifice plates are installed within the flow channels of a boiler wind box assembly.
In the wind box assembly, as partly shown in perspective in FIG. 6, air for combustion enters from the upper right and passes through a bend 11 to a wind box entrance duct 12.
Separate wind boxes for air supply to burners are indicated at 13. The air for the burners is fed from inlets 15 through the burners into the boiler furnace.
The numeral 14 designates other wind boxes for overfire air supply. Air is forced through inlets 16 into the furnace to serve as over-fire air.
FIG. 7 is a transverse section through the wind box entrance duct 12, wind boxes 14, and inlets 16. Dampers for controlling the over-fire air supply are indicated at 17.
By way of example, the use of segmental orifice plates as the primary flow-measuring means for the automatic control of the flow rate of over-fire air will be described hereunder.
Because of the burner arrangement in the boiler, the ductwork for over-fire air supply consists of two duct systems, on the left and right sides in the form of wings A and B as in FIG. 7. The two systems are asymmetrical not only to each other, as can be seen from FIG. 7, but also to the wind box bend 11, as shown in FIG. 6. Thus, it will be readily appreciated that, if primary measuring elements are separately installed in the two systems, the different upstream configurations of the ducts will cause the elements to make dissimilar or unrelated errors.
With a wind box assembly of the construction described, it is a primary consideration to achieve equal measurement accuracy at the wings A and B, as with other wind box constructions. A matter of secondary consideration is to help the dampers function as efficiently as possible. Thirdly, the pressure loss relative to the differential pressure produced must be minimized.
Next, the segmental orifice plate installation will be described. FIG. 8 is an enlarged vertical section of the wing B duct of FIG. 7. FIG. 9 is a section through the line IX--IX of FIG. 8. Both figures illustrate a conventional segmental orifice plate 18 installed inside the duct, on the side d.
On condition that the same ratio of the orifice cross sectional area to the channel cross sectional area (m = 0.6) be maintained, the corresponding segmental orifice plate was attached to the side a, c, or d instead of on the side b, and each time the error was determined. Pressure taps 19a, 19b were formed in tandem along the center of each side, upstream and downstream of the orifice plate, and the respective pressures were taken out to obtain the differential pressure.
FIG. 10 is a graphical representation of the errors observed in the wings A and B with different segmental orifice plate locations. The errors on the ordinate are those in the differential pressures thus produced, given in percent on the basis of the differential pressures (normal, reference values) obtained from duct sides having adequate straight sections upstream, the reference values being 100.
It will be seen from these data that the errors in differential pressures produced vary, and largely, with the orifice plate locations, and that there are material discrepancies between the errors in the wings A and B.
FIG. 11 is a graph showing the relationship between the differential pressure produced by the segmental orifice plate secured to the side d and the flow velocity of the fluid. It indicates that the error in differential pressure depends also upon the Reynolds'number Re (flow velocity) in the closed channel.
From the foregoing discussion it is clear that the use of existing segmental orifice plates as primary means for measuring the flow rate in boiler wind boxes is not desirable for the purpose of automatic control of the flow of over-fire air for the boiler.